The present invention generally relates to spray booths of use in applying coatings to desired substrates. More particularly, the present invention relates to a spray booth for applying coatings to substrates under conditions where the temperature and/or humidity of the environment established within the spray booth must be controlled to ensure coating quality.
Various types of substrates are coated with various types of coatings, and in each case, the environment surrounding the substrate must be effectively controlled to ensure the quality of the coating. For example, automotive parts, aerospace parts, and appliance parts have various types of coatings applied to them. Coatings such as paints, base coats, top coats, and aqueous metallic slurries may be applied to these parts to achieve various benefits.
As an example, U.S. Pat. No. 3,248,251 (Allen) discloses metal filled aqueous chromate/phosphate slurries. These slurries are commonly used on aerospace parts to impart a desired quality finish to the surface of the part. The quality of the finish is directly related to the environment in which the part is coated.
The quality of any finish (of an applied paint or other coating) depends upon the cleanliness of the environment in which it is applied. With regard to waterborne materials, such as are described in U.S. Pat. No. 3,248,251 the moisture content of the environment can also exert a significant influence upon the affecting deposition rate, drying rate and more. For this reason it is often important to control humidity and/or temperature of the environment in which a coating is being applied.
There are also environmental concerns related to the application of such coatings. For example, chromate/phosphate slurries and aluminum filled chromate/phosphate coatings are widely used in aerospace applications. The chemical stability of the slurry composition and the corrosion resistance of the binder system of these coatings are a consequence of the presence of hexavalent chromium in the material. Hexavalent chromium is environmentally toxic and its levels must be controlled during application. Control is particularly critical when the coating films are deposited on parts by air spray techniques.
It was therefore desirable to provide a spray booth for applying coatings such as those discussed above, which spray booth could control the environment around the part being coated. This would serve to facilitate the reproducible deposition of uniform, tightly adherent, smooth coatings.
Early spray booths which controlled the spraying environment were expensive and inefficient devices in which large volumes of fluid (especially air) were subjected to heating, cooling, and humidity control, as appropriate for a particular application and moved through the working area at high speeds.
To minimize the amount of work the fluid was typically recirculated in these devices. However, this recirculated air passed by and around the operator, resulting in problems with Federal air/workplace regulations.
Recognizing this, efforts were made to reduce the cost of controlling temperature and humidity in the working fluid by limiting the amount being treated to that only in the vicinity of the part being coated. Ambient air was passed through a humidity and/or temperature control apparatus then directly over the substrate to be coated, creating a controlled region or curtain of condition of a size sufficient to surround the part being coated, but less than the total size of the room receiving such equipment.
To this end, U.S. Pat. No. 5,127,574 (Mosser et al.) discloses a spray booth for applying coatings to substrates which is defined by an outer chamber containing a fluid (such as air) at a first temperature and/or humidity, and an inner chamber positioned within and in fluid communication with the outer chamber, capable of handling the fluid to develop a limited and controlled environment for receiving substrates to be coated.
The inner chamber is caused to operate continuously at a second humidity and/or temperature higher than that of the temperature and/or humidity in the outer enclosure. This is achieved by recirculating the flow of fluid in the inner chamber and continuously adjusting its temperature and/or humidity to a predetermined level.
Appropriate filters remove airborne contaminants from the recirculating fluid. Means are also provided for exhausting desired amounts of the fluid from the inner chamber to the environment external of the outer chamber.
In operation, the outer chamber is maintained at a first temperature and/or humidity responsive to the operation of appropriate air handling equipment (which can include air cleaning equipment if desired). Fluid is drawn from this outer chamber, into the inner chamber to modify the temperature and/or humidity of the fluid handled by the inner chamber (to achieve the second temperature and/or humidity) at a rate equal to or greater than the rate at which fluid is exhausted from the inner chamber to the environment external to the outer chamber.
The system disclosed in U.S. Pat. No. 5,127,574 provides better control of the process environment around a component, especially as relates to spraying a coating in air, than possible in any of the prior art, provided that:
there is an outer enclosure in which conditions of the process fluid (air), for example, its temperature and humidity, are precisely controlled; and PA1 the part to be processed is completely encapsulated within the working fluid being recirculated within the inner loop.